Dark posting allows one to share stories with audiences in a specific city, state, or region without having to post directly to one's Facebook wall. Dark posts do not have to appear on a Page wall, yet they may reach audiences on Facebook in the Newsfeed (with very precise targeting and messaging).
One may have multiple posts live at the same time targeting different audiences in order to optimize for the posts, photos, and copy that resonate the most with a reader.
For example for this piece produced for Marmot, dark posts were used to create multiple ads that targeted different audiences. Those were optimized for the post that performed the best.
For more information about Facebook Ads, visit this page.